Curbing the emission of vent gases contaminated with organic vapors is an urgent environmental problem. Of particular concern are streams containing halogenated hydrocarbons, frequently used as solvents, or chlorofluorocarbons (CFCs), which deplete the earth's protective ozone layer. Treatment methods may be chemical, in which case the organic substance is destroyed completely or converted to another form, or physical treatment, in which case the waste is changed in volume or composition, and there is an opportunity for recovery of the organic. A variety of separations or treatment technologies are described in the literature, yet few are in use, either because the technology has not been developed to the industrial level, or because the process is too costly for general application. The cost of most treatment processes is concentration dependent. The widely used processes are carbon adsorption, incineration and compression condensation. Carbon adsorption becomes expensive above about 0.1-0.5% organic content in the stream to be treated, because the plant operating and capital costs increase with increasing solvent concentration. Also, high concentration leads to unacceptably high temperatures in the carbon bed, because the adsorption step is exothermic. Consequently process streams must frequently be diluted many-fold before being passed to the carbon beds.
Incineration, because it involves burning the organic vapor with a supplementary fuel, such as natural gas, is best suited to relatively concentrated streams. Incineration is relatively costly when the concentration of organic in the stream is below about 5-10%. Neither carbon adsorption nor incineration is easily conducive to organic vapor recovery, and both can create secondary waste or pollution problems; in the case of carbon adsorption, spent contaminated carbon from the beds, and in the case of incineration, large volumes of carbon dioxide or other combustion products.
It is, therefore, highly desirable to limit the quantities of organics that need to be treated by these methods. Prior to discharge or treatment, many organic-containing gas streams are sent to condensation or compression-condensation systems. In these systems, the gas is chilled to below its dewpoint temperature, with or without prior compression. With appropriate compounds, a substantial amount of the organic condenses out and can be recovered. Problems encountered in condensation are (1) low organic concentration in the stream, and/or low boiling point, so that the dewpoint is difficult to reach, and (2) formation of ice in the condenser brought about by water vapor in the feedstream. Condensation becomes expensive and impractical if the organic content in the stream to be treated is below about 20-30% of the saturation concentration at room temperature and pressure. For condensation to be economically viable, a feed concentration as much as 40-50% is frequently required.
There is, thus, a treatment gap in the concentration range at least above about 0.1-0.5% and below about 5-50%, depending on the compound, which the existing technologies are ill suited to fill. Consequently organic-laden streams in this concentration range must often be diluted or concentrated before they can be fed to the treatment process. An efficient, economic separation process, capable of handling streams in this range at source, would represent a valuable addition to pollution control technologies.